Display components in image display devices, such as television devices, are now shifting from conventional cathode-ray tube displays to thin display panels, such as liquid crystal panels and plasma display panels. With the thin display panels, the thicknesses of the image display devices can be reduced. Liquid crystal panels included in the liquid crystal display devices do not emit light, and thus backlight devices are required as separate lighting devices. The backlight devices are generally classified into direct-type and edge-light-type according to mechanisms. An edge-light-type backlight device includes a light guide plate for guiding light from a light source and an optical member for converting the light from the light guide plate to even planar light with optical properties and for supplying the light to a liquid crystal panel. An example of such a device is disclosed in Patent Document 1. Patent Document 1 discloses a configuration that includes multiple ridged lenses on a light exiting surface of a light guide plate such that the light guide plate has light collecting properties to improve brightness without a prism sheet.